1. Field of the Invention
The present invention generally relates to a multiple chamber semiconductor wafer processing system and, more specifically, the present invention relates to a method and apparatus for accessing a multiple chamber semiconductor wafer processing system.
2. Description of the Related Art
In integrated circuit (IC) manufacturing systems that handle 200 mm wafers, the interface to the factory is a 25-wafer load-lock. The load-lock forms an entry and exit point for the IC manufacturing system. In use, a cassette carrying 25 wafers is placed into a load-lock and the air in the load-lock is removed to form a vacuum in the load-lock. The wafer cassette is vertically positioned in the load-lock to align a particular wafer with a wafer transport mechanism, e.g., a wafer transport robot.
The time required to produce a vacuum in the load-lock is referred to herein as Tpump. After processing within the IC manufacturing system, the processed wafer is returned to the cassette. When all the wafers have been processed and returned, the air in the load-lock is then let into the load-lock at a controlled rate in what is referred to as a vent operation. The time required to perform a vent operation is referred to herein as Tvent. Once venting is complete, the cassette is removed from the load-lock. The maximum achievable load-lock throughput is represented as:       S    LL    =            2      ⁢              xe2x80x83            ⁢      K                      T        pump            +              T        vent            +              T        load            +              T        unload            
where:
K is the maximum number of wafers in the load-lock;
Tpump is the time required to evacuate the load-lock;
Tvent is the time required to fill the load-lock with air;
Tload is the time required to load a cassette into the load-lock; and
Tunload is the time required to unload a cassette from the load-lock.
Load-locks are generally double-buffered by using a pair of parallel load-locks that feed wafers to a wafer processing system. As such, loading, pumping, venting, and unloading of one cassette is overlapped with processing of wafers from another cassette. Without double-buffering, no overlap is possible and the time required to pump and vent a load-lock can be almost as long as the wafer processing time.
FIG. 1 depicts a semiconductor wafer processing system or cluster tool 100 for 300 mm wafers comprising a plurality of process chambers 1101, 1102, 1103, and a transfer chamber 112. The cluster tool 100 is coupled to a factory interface 104. The FI 104 comprises a pair of single wafer load-locks (SWLLs) 102A and 102B, FI transfer space 105 containing a wafer transport agent 106, and at least one load port (two are depicted as ports 108A and 108B). The FI 104 may optionally comprise a wafer orientor, a pass-through slot, one or more cool down positions, a metrology station, or defect control station.
A tool transfer agent 114 (commonly referred to as a robot) accesses the chambers 1101, 1102, 1103 and the load-locks 102A, 102B to move wafers amongst the chambers and load-locks. The SWLL 102A and 102B are used as the entry and exit points to the tool 100. The load-locks 102A and 102B each retain only one wafer at a time during the pump/vent cycle.
The FI load ports 108A and 108B are supplied with wafer cassettes that hold up to 25 wafers each. When a transfer agent 106 having a single wafer transport blade is used in either the tool 100 or in the FI 104, the SWLLs have two slots each. Using two slots enables a single blade robot (SBR) to wait for an SWLL to either pump or vent while being pre-positioned in front of the SWLL with a wafer on its blade. The robot then puts the wafer in an empty slot first before taking a wafer out of the SWLL. With two slots, the load and unload times are substantially decreased. However, the load-lock volume is increased to accommodate the second slot which increases pump and vent times.
When dual blade robots are used in both the tool 100 and the FI 104, then only a single slot SWLL is used and load-lock volume is reduced. The second blade serves as a buffer that accepts a wafer from the SWLL before placing a new wafer from the other blade of the robot into the SWLL.
Wafers from the FI load ports 108A or 108B are directed toward either SWLL 102A or 102B depending upon which SWLL is available to be loaded. If both SWLLs are available, the wafer enters the nearest SWLL to the current position of the transfer agent 106. Wafers leaving the transfer chamber 112 of the tool 100 are directed into either SWLL 102A or 102B depending upon which SWLL is available. If both SWLL are available, the wafer enters the one nearest the present position of the tool transfer agent 114. The FI 104 returns the wafer to the source cassette into its original position, i.e., preserving slot integrity. Wafers that enter the transfer chamber 112 through SWLL 102A are not restricted to exiting the transfer chamber 112 through SWLL 102A. Similarly, wafers from one cassette can enter either SWLL 102A or 102B depending on the load-lock availability. In other words, unless explicitly restricted by the system controller software, wafers from one cassette are not restricted from entering and leaving the tool via a particular load-lock.
The fixed nature of the number of slots to either one or two can result in substantial transfer delays for wafers entering and leaving a cluster tool. Therefore, there is a need in the art for a method and apparatus to improve wafer throughput in semiconductor wafer processing systems that use load-locks.
The present invention is a factory interface for a semiconductor wafer processing cluster tool having a K-wafer load-lock (KWLL) to facilitate accessing a multiple chamber semiconductor wafer processing system. The KWLL comprises a variable number of K+1 wafer slots assigned as inbound and outbound slots. Inbound slots are used to send up to K+1 wafers into the cluster tool and the same physical slots, denoted as outbound slots, are used for receiving up to K+1 wafers from the cluster tool. The K+1 slots are in the same volume that has to be pumped for wafers to enter the tool and vented for wafers to leave the tool. These K+1 slots accommodate up to K wafers when accessed by a single blade robot from the tool or the factory interface, and up to K+1 wafers when the tool and factory interface are equipped with dual blade robots. Various KWLL loading methods can be selected to optimize the throughput of a wafer processing system using the KWLL. Such methods illustratively include a wafer packing method, a reactive method and a gamma tolerant method.